Geology Reference
In-Depth Information
10
Fission-track
. The spontaneous nuclear fission
of uranium-238 damages uranium-bearing min-
erals such as apatite, zircon, sphene, and glass.
The damage is cumulative. Damaged areas can
be etched out of the crystal lattice by acid,
and the fission tracks counted under a micro-
scope. The density of tracks depends upon the
amount of parent isotope and the time elapsed
since the tracks were first preserved, which only
starts below a critical temperature that varies from
mineral to mineral.
11
Luminescence
. This is a measure of the
background radiation to which quartz or
feldspar crystals have been exposed since
their burial. Irradiated samples are exposed to
heat (thermoluminescence - TL) or to par-
ticular wavelengths of light (optically stim-
ulated luminescence - OSL) and give off
light - luminescence - in promotion to the total
absorbed radiation dose. This in turn is propor-
tional to the age. Suitable materials for dating
include loess, dune sand, and colluvium.
12
Electron-spin resonance
. This method measures
the accumulation of electrical charges in crystal
lattice defects in silicate minerals resulting from
natural radiation.
Geomorphological methods
These methods gauge the cumulative results of com-
plex and interrelated physical, chemical, and biological
processes on the landscape.
16
Soil-profile development
. A method that uti-
lizes systematic changes in soil properties owing
to weathering and pedogenic processes. It uses
measures of the degree of soil development,
such as A-horizon thickness and organic con-
tent, B-horizon development, or an overall Profile
Development Index.
17
Rock and mineral weathering
. A method that
utilizes the systematic alteration of rocks and
minerals owing to exposure to weathering agents.
18
Scarp form
. A method based on the progressive
change in scarp profile (from steep and angu-
lar to gentle and rounded) resulting from surface
geomorphic processes.
Correlation methods
These methods substantiate age equivalence using
time-independent properties.
19
Palaeomagnetism
. Some minerals or particles
containing iron are susceptible to the Earth's mag-
netic field when heated above a critical level -
the Curie temperature. Minerals or particles in
rocks that have been heated above their critical
level preserve the magnetic-field alignment pre-
vailing at the time of their formation. Where
the rocks can be dated by independent means,
a palaeomagnetic timescale may be constructed.
This timescale may be applied elsewhere using
palaeomagnetic evidence alone.
20
Tephrochronology
. This method recognizes
individual tephra (p. 12) by their unique proper-
ties, and correlates them with a dated chronology.
21
Palaeontology
. This experimental method uses
either the progressive evolution of a species or
shifts in zoogeographical regions.
22
Climatic correlations
. This method correlates
landforms and deposits to global climatic changes
of known age.
Chemical and biological methods
These
methods
measure
the
outcome
of
time-
dependent chemical or biological processes.
13
Amino-acid racemization
. This method is based
upon time-dependent chemical changes (called
racemization) occurring in the proteins preserved
in organic remains. The rate of racemization is
influenced by temperature, so samples from sites
of uniform temperature, such as deep caves, are
needed.
14
Obsidian hydration
. A method based upon the
increase in thickness of a hydration rind on an
obsidian surface.
15
Lichenometry
. A method based upon the growth
of lichens on freshly exposed rock surfaces. It may
use the largest lichen and degree of lichen cover
growing on coarse deposits.
